There are two industry standards used to communicate digital picture data from radars to displays: ARINC 453 and WXPD.
ARINC 453 is an industry standard mutually developed by radar and display manufacturers in the late 1970's. It was never released as an ARINC standard, but was adopted in the ARINC 708 standard.
The two standards have some similarities in the physical layer in that they both use Manchester II encoded bits at a 1 Mbps data rate, which is self-clocking and so only requires one twisted-shielded pair per display in the aircraft installation.
However, the formatting of the data between the two standards is different. ARINC 453 radials contain a 64-bit preamble including information such as radar mode, tilt, gain, BITE status and the angle of the radial at which the picture data should be drawn. The picture data then follows this 64 bit preamble in the form of 512 3-bit words corresponding to ranges from 0 to the indicated display range, this range being divided into 512 increments. For example, on the 320 nm display range, each 3-bit word corresponds to 320/512ths nautical miles. The 3 bits in each word are encoded with color data, allowing for up to 8 colors, e.g. black, green, yellow and red (some manufacturers allow additional colors).
Hence, with ARINC-453, a 1600-bit packet (64 bits of preamble+512*3 color words) is sent from the radar to the display for every radial line desired to be drawn on the display, hence the picture is drawn in the manner of a “windshield wiper.” See FIG. 1.
By contrast, the WXPD data was originally intended to only communicate picture data and does not include a preamble (the equivalent data is sent on a separate lower-speed RS-422 bus known as Serial Control Interface or SCI). It does however contain an 8 bit start sequence to sync clocks and a 3-bit invalid Manchester sequence to alert the start of a new radial. Additionally, instead of specifying a radial angle at which the display is to draw the radial, WXPD is arranged as several 5-bit words, typically 256 (more or less are possible) in which case there typically would be a total of 1280 bits of payload per radial transmission. Each 5-bit word contains two bits, referred to as Inc-X and Inc-Y, and three color bits (similar in that respect to ARINC 453). The Inc-X and Inc-Y bits are used to specify the direction relative to the previously affected display pixel in which the next pixel is to be drawn.
The display is assumed to be organized as a grid of 256×256 pixels as shown FIG. 2. The coordinate of each pixel is given in terms of an X value and Y value. The Y-values range from 0 to 255, whereas the X values range from 0 to 127, but can be either right or left of the origin (corresponding to “positive” or “negative” values along the X-axis). The display maintains two values internal registers corresponding to the Y address and X address of the pixel to be displayed.
At the beginning of the radial packet is a sequence of 8-bits of Manchester II encoded zeros, which syncs up the display's clock with the data packet it is receiving. Then there is a 3-bit invalid Manchester II word sequence which causes the display to reset the two pixel position registers to X=0, Y=0, positioning the current pixel position at the bottom center of the WXPD display. The first 5-bit word contains status data for setup of the display, including determining which direction along the X-axis the X register represents. The second word is designated as reserved and is not used. The display assumes the color information for these two words to be black. The third 5-bit word begins the radial picture data. Upon receiving a 5-bit word, if the Inc-Y bit is set and the Inc-X bit is not, the display hardware increments the Y address counter, so that the resulting address of the pixel is [0,1], that is, 1 pixel in the Y direction and 0 pixels in the X direction. The display then writes the appropriate color at that pixel as determined by the remaining 3 bits in the 5-bit word.
Likewise, if the Inc-X bit is set and they Inc-Y bit is not, the display hardware increments the X address counter so that the resulting address of the pixel is [1,0], that is, 1 pixel in the X direction and 0 pixels in the Y direction. The display then writes the appropriate color at that pixel. The information as to whether the X address is to be interpreted as left or right is encoded in the first 5-bit word of the radial packet. Every Inc-X bit in a given WXPD radial is interpreted as an increment to either left or right as determined by that first status word.
If both Inc-X and Inc-Y are set, the display will actually increment the Y address (X=0 and Y=1), write the color and then increment the X-address (X=1 and Y=1) and write the same color, hence there are two writes in this case for the single 5-bit word.
Three examples are shown in FIG. 3. In this simplified example, the display is 16×16 instead of the normal 256×256).
For Case A, only the Inc X bits are set (assumes first 5 bit word indicates X addresses to be interpreted as “right”). (Also note display hardware limits addresses as appropriate.) For Case B only the Y-bits are set and in Case C, the Inc-X, Inc-Y or both are set so as to result in radial at a given angle being drawn.
For Case C, where there are two adjacent pixels in a given row, the 5-bit word has both Inc-X and Inc-Y set and so internally the display increments the Y address, writes the color data then increments the X address and writes the same color data. This feature helps ensure no “holes” in the displayed data. It is sometimes referred to as the “fill algorithm.”
In the intended use of WXPD, the radar would draw radial lines at angles corresponding to the current antenna angle. This obviates the need for the radar to store an entire 2D memory of data, that is, it can communicate the data it collected in real time in its native (polar) coordinates to the display.
It is possible however to draw other than straight lines (or at least their pixilated equivalents). Any shape of line that can be described with monotonically increasing values of the (magnitude of) X and Y addresses can be drawn, like 7's, L's or steps.
The EGPWS system has taken advantage of this fact in WXPD displays to draw a series of vertical lines that sweep from side to side. This is often called “curtain mode” and is illustrated in FIG. 4.
These vertical lines are realized by sending N 5-bit words with only Inc-X set, where N corresponds to the desired absolute offset from the X=0 coordinate of the vertical line. These are followed by 256 5-bit words with only Inc-Y set to “draw” a line from the bottom to the top of the display. These result in a series of “L's” (or backwards “L's”) being written to the display, where the horizontal leg of the “L” is varied in length.
This method greatly simplifies the processing of data natively Cartesian for display by eliminating the trigonometry generally involved in converting the angle to a series of Inc-X and Inc-Y values. This is in fact is very important when text is embedded in the WXPD display data. The traditional radial method makes it very difficult to ensure a one-to-one correspondence between the source Cartesian data and the resulting displayed data due to quantization effects.
It has a second advantage of optimizing the update rate of the display since there is less redundant data sent. In the radial mode, generally 512 radials are required to cover the entire display (much of the data near the origin is written over by subsequent radials). In contrast, curtain mode can cover the entire display in 256 radials.
A typical “curtain mode” EGPWS display is shown in FIG. 5. The EGPWS display shown is in “peaks” mode where the top number (096) in the lower right hand corner represents the high terrain elevation displayed and the lower number (025) represents the altitude of the boundary between amber indications and black. These numbers are actually embedded in the WXPD picture data as bit-mapped images.
The RDR-4000 IntuVue™ radar, by Honeywell International Inc., incorporates new operating modes for which the appropriate annunciations are not supported by many legacy WXPD or ARINC 453 displays. Two specific examples are that the RDR-4000 has an Automatic Weather Mode and a Manual Weather Mode. Legacy WXPD displays do not provide a means to annunciate to the crew which of these two modes is selected. Further, there is a crew selectable parameter associated with the Manual Weather Mode, specifically, altitude, which sets the altitude “slice” to be displayed.
The text-embedded-in-picture data concept used by EGPWS can be used to solve these problems. However, while the crew is adjusting the altitude parameter, feedback in the form of a displayed value is updated only once every 3-4 seconds which is an inadequate delay from a human factors standpoint.
It is not possible to increase the update rate of the entire display due to an inherent limit of 240 radials per second (current implementations self impose a more practical limit of approximately 120 Hz because WXPD displays do not necessarily adhere exactly to the engineering specification for the display bus). Thus, the only solution is an expensive one and that is to replace the aircraft's cockpit with a new display system or update the embedded software in the legacy display—both of which are normally cost prohibitive.